Today's automobiles have increasingly stringent government-mandated fuel consumption and exhaust emission standards. Manufacturers are utilizing more powerful engine controllers and control algorithms to optimize the engine combustion process. In order to precisely meter fuel-to-air ratio, control idle speed, and provide good vehicle acceleration, a good estimate of engine load is needed.
A belt-driven automotive air conditioning compressor, which utilizes power originating from the vehicle engine to cool a vehicle's interior compartment, is a significant load requiring an accurate estimation.
Compressor power is a function of compressor speed and torque. While compressor speed can be easily derived from measured engine speed with a known constant pulley ratio, compressor torque cannot be measured cost-effectively in a production vehicle and thus requires estimation by an algorithm in the Engine Controller.
Historically, adjustments in vehicle engine performance are made by the vehicle engine controller from a calculation of compressor torque or compressor power. One typical method of implementing the load calculation is to use an equation or look up table which is a function of compressor speed and discharge pressure. This equation is appropriate for steady-state operation of a fixed-displacement compressor or for steady-state operation of a variable-displacement compressor at ambient conditions requiring full capacity.
A typical torque equation has limitations predicting torque during transient conditions or when the variable compressor is at partial capacity. The typical equation sometimes over predicts the compressor torque during transients and at partial capacity which results in decreased fuel consumption.
Power estimation equations for fixed and variable displacement compressors are known in the art. Equation 1 below is appropriate for steady state conditions and full capacity although the steady state accuracy may be improved. Another known method utilizes a torque prediction look-up table as a function of discharge pressure and valve current. Again, the look-up table is appropriate for steady state and full capacity conditions. Steady-state accuracy is improved, but there is still a need for greater steady-state accuracy.Power=A+B*Pressure+C*Pressure2+D*Pressure3−E*CRPM+F*Pressure*CRPM,  Equation 1
where                Pressure is in Bar Gage        Power is in Watts        CRPM is compressor revolutions per minute        The applicable range of this equation is from 100 to 6100 Watts.        
For compressor 1: A=10                B=88        C=−17.375        D=0.921875        E=0.37390136        F=0.07336425        
For compressor 2: A=0                B=46.9        C=0        D=0        E=0.4881        F=0.1056        
U.S. Patent Application Publication US 2010/0236265 A1 published 23 Sep. 2010 to Higuchi et al. entitled Air Conditioner for Vehicle, describes a torque estimation strategy with a set of inputs materially different from the present invention. U.S. Publication US 2010/0236265 A1 and its related foreign counterparts are hereby incorporated herein by reference.
As described by U.S. Publication US 2010/0236265 A1, an air conditioner for a vehicle comprises a refrigeration cycle including a variable displacement compressor for refrigerant which uses an engine as a drive source, a condenser, an evaporator, a displacement adjuster means for outputting an adjustment control signal to the compressor, and a compressor torque calculation means for calculating the torque of the compressor. The compressor torque calculation means includes at least two torque estimation means of a saturation region torque estimation means corresponding to a case where the compressor is driven at a maximum discharge displacement and a displacement control region torque estimation means corresponding to a case where it is driven at a discharge displacement other than the maximum discharge displacement, and also includes a correction means for correcting the calculation of the torque of the compressor when a change in engine rotational speed greater than a set value is detected. Purportedly, even when the engine rotational speed changes rapidly, the torque of the compressor in the refrigeration cycle can be calculated.
The methodology described in U.S. Publication US 2010/0236265 A1 is based on a torque calculation utilizing the following inputs:                Displacement control signal        Outside air temperature        Compressor rotational speed        Vehicle running speed        Condenser fan voltage        High side refrigerant pressure        Blower voltage        
The methodology described in U.S. Publication US 2010/0236265 A1 is highly theoretical and fails to adequately instruct an expert in the art how to practice the invention.